1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly, to an image reading apparatus which images image information of an original on a charge coupled device (CCD) through an imaging lens and causes the CCD to read the image information.
2. Description of the Related Art
There is conventionally known an image reading apparatus which reads image information of an original and the like with a CCD sensor for three RGB colors, or the like by line-scanning in a sub-scanning direction (see Japanese Laid-Open Patent Publication (Kokai) No. H06-113112, for example).
Such an image reading apparatus may have an SLA (SELFOC (Trade Mark) lens array) used as an imaging lens in many cases. It is generally known that the SLA has a characteristic of a narrow depth of field.
This image reading apparatus comprises an original glass plate on which an original is placed, a shaft disposed below the original glass plate in parallel thereto and directed in a sub-scanning direction, and an optical unit which moves on this shaft. The optical unit comprises an illumination light source which illuminates the original placed on the original glass plate, a CCD sensor, and an imaging lens which images the reflected light from the original placed on the original glass plate on the CCD sensor.
From the viewpoint of the construction of this image reading apparatus, an image resolution (CTF values) cannot be adjusted in a state that the optical unit is mounted on to a main body of the image reading apparatus body, and hence, severe management of a standard of a thickness of the original glass plate used causes deterioration of the image resolution to be reduced.
However, when the optical unit is exchanged during normal use, there is a problem that the variable thickness of the original glass plate based on an individual difference between the image reading apparatuses causes the imaging position to shift, thereby allowing the image resolution (CTF values) to change between before and after exchange of the optical unit.
A change of the CTF values due to such exchange of the optical units causes deterioration of the color reproducibility of the original, particularly in color.